Natural starch is a readily available and inexpensive material. Therefore, attempts have been made to process natural starch on standard equipment using existing technology known in the plastic industry. However, since natural starch generally has a granular structure, it needs to be “destructurized” and/or otherwise modified before it can be melt-processed like a thermoplastic material. The task of spinning starch materials to produce fine-diameter starch fibers, or more specifically, the fibers having average equivalent diameters of less than about 20 microns, suitable for production of tissue-grade fibrous webs, such as, for example, those suitable for toilet tissue, presents additional challenges. First, the processable starch composition must possess certain rheological properties that allow one to effectively and economically spin fine-diameter starch fibers. Second, it is highly desirable that the resulting fibrous web, and therefore the fine-diameter starch fibers comprising such a web, possesses a sufficient wet tensile strength, flexibility, stretchability, and water-insolubility for a limited time (of use).
“Thermoplastic” or “thermoplastically-processable” starch compositions, described in several references herein below, may be suited for production of starch fibers having good stretchability and flexibility. The thermoplastic starch, however, does not possess the required wet tensile strength which is a very important quality for such consumer-disposable articles as toilet tissue, paper towel, items of feminine protection, diapers, facial tissue, and the like.
In the absence of strengthening agents, such as, for example, a high level of relatively expensive water-insoluble synthetic polymers, cross-linking may be necessary to obtain a sufficient wet tensile strength of starch fibers. At the same time, chemical or enzymatic agents have been typically used to modify or destructurize the starch to produce a thermoplastic starch composition. For example, a mix of starch and a plasticizer can be heated to a temperature sufficient to soften the resulting thermoplastic starch-plasticizer mix. In some instances pressure can be used to facilitate softening of the thermoplastic mix. Melting and disordering of the molecular structure of the starch granule takes place and a destructurized starch is obtained. However, the presence of plasticizers in the starch mix interferes with cross-linking of the starch and thus discourages the resulting starch fibers from acquiring a sufficient wet tensile strength.
Thermoplastic or thermoplastically-processable starch compositions are described in several U.S. patents, for example: U.S. Pat. No. 5,280,055 issued Jan. 18, 1994; U.S. Pat. No. 5,314,934 issued May 24, 1994; U.S. Pat. No. 5,362,777 issued November 1994; U.S. Pat. No. 5,844,023 issued December 1998; U.S. Pat. No. 6,117,925 issued Sep. 12, 2000; U.S. Pat. No. 6,214,907 issued Apr. 10, 2001; and U.S. Pat. No. 6,242,102 issued Jun. 5, 2001, all seven immediately preceding patents issued to Tomka; U.S. Pat. No. 6,096,809 issued Aug. 1, 2000; U.S. Pat. No. 6,218,321 issued Apr. 17, 2001; U.S. Pat. No. 6,235,815 and U.S. Pat. No. 6,235,816 issued on May 22, 2001, all four immediately preceding patents issued to Lorcks et al.; U.S. Pat. No. 6,231,970 issued May 15, 2001 to Andersen et al. Generally, the thermoplastic starch composition can be manufactured by mixing starch with an additive (such as a plasticizer), preferably without the presence of water as described, for example, in U.S. Pat. No. 5,362,777 referenced herein above.
For example, U.S. Pat. Nos. 5,516,815 and 5,316,578 to Buehler et al. relate to thermoplastic starch compositions for making starch fibers from a melt-spinning process. The melted thermoplastic starch composition is extruded through a spinneret to produce filaments having diameters slightly enlarged relative to the diameter of the die orifices on the spinneret (i.e., a die swell effect). The filaments are subsequently drawn down mechanically or thermomechanically by a drawing unit to reduce the fiber diameter. The major disadvantage of the starch composition of Buehler et al. is that it requires significant amounts of water-soluble plasticizers which interfere with cross-linking reactions to generate apparent peak wet tensile stress in starch fibers.
Other thermoplastically processable starch compositions are disclosed in U.S. Pat. No. 4,900,361, issued on Aug. 8, 1989 to Sachetto et al.; U.S. Pat. No. 5,095,054, issued on Mar. 10, 1992 to Lay et al.; U.S. Pat. No. 5,736,586, issued on Apr. 7, 1998 to Bastioli et al.; and PCT publication WO 98/40434 filed by Hanna et al. published Mar. 14, 1997.
Some of the previous attempts to produce starch fibers relate principally to wet-spinning processes. For example, a starch/solvent colloidal suspension can be extruded from a spinneret into a coagulating bath. References for wet-spinning starch fibers include U.S. Pat. No. 4,139,699 issued to Hernandez et al. on Feb. 13, 1979; U.S. Pat. No. 4,853,168 issued to Eden et al. on Aug. 1, 1989; and U.S. Pat. No. 4,234,480 issued to Hernandez et al. on Jan. 6, 1981. JP 08-260,250 describes modified starch fibers manufactured from starch and an amino resin precondensate, and a method for making the same. The method includes dry spinning of an undiluted solution of starch and amino resin precondensate, followed by heat treatment. The starch used in this application is natural starch, such as contained in corn, wheat, rice, potatoes etc.
The natural starch has a high weight average molecular weight—from 30,000,000 grams per mole (g/mol) to over 100,000,000 g/mol. The melt-rheological properties of an aqueous solution comprising such starch are ill-suited for high-speed spinning processes, such as spun-bonding or melt-blowing, for production of fine-diameter starch fibers.
The art shows a need for an inexpensive and melt-processable starch composition that would allow one to produce fine-diameter starch fibers possessing good wet tensile strength properties and suitable for production of fibrous webs, particularly tissue-grade fibrous webs. Consequently, the present invention provides non-thermoplastic fine-diameter starch fibers having sufficient apparent peak wet tensile stress. The present invention further provides a process for making such non-thermoplastic starch fibers.